tools/bpftool: show btf id in program information

Let's add a way to know whether a program has btf context.
Patch adds 'btf_id' in the output of program listing.
When btf_id is present, it means program has btf context.

Sample output:
user@test# bpftool prog list
25: xdp  name xdp_prog1  tag 539ec6ce11b52f98  gpl
	loaded_at 2019-04-10T11:44:20+0900  uid 0
	xlated 488B  not jited  memlock 4096B  map_ids 23
	btf_id 1
Signed-off-by: Prashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/bpf/bpftool/prog.c

@@ -249,6 +249,9 @@ static void print_prog_json(struct bpf_prog_info *info, int fd)
 	if (info->nr_map_ids)
 		show_prog_maps(fd, info->nr_map_ids);
 
+	if (info->btf_id)
+		jsonw_int_field(json_wtr, "btf_id", info->btf_id);
+
 	if (!hash_empty(prog_table.table)) {
 		struct pinned_obj *obj;
 
@@ -319,6 +322,9 @@ static void print_prog_plain(struct bpf_prog_info *info, int fd)
 		}
 	}
 
+	if (info->btf_id)
+		printf("\n\tbtf_id %d\n", info->btf_id);
+
 	printf("\n");
 }